The use of optical communications involving the use of optical fibers has grown at an unprecedented pace. Typically, an optical fiber has a diameter on the order of 125 microns, for example, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example. Optical cables may comprise a plurality of these optical fibers which are stranded together or which are assembled in planar arrays which are referred to as ribbons.
The technology for forming low-loss optical fibers, which is shown for example in U.S. Pat. No. 4,217,027 which issued on Aug. 12, 1980 in the names of J. B. MacChesney and P. B. O'Connor, has advanced to a point where there is widespread commercial manufacturing of optical fibers. Most processing includes drawing an optical fiber from a previously manufactured glass boule, sometimes referred to as a preform. After it has been drawn, the optical fiber is usually provided with a layer of a protective curable coating material which may be cooled or cured thermally, by radiation, or by other suitable techniques for achieving solidification.
The introduction of optical fiber applications to evermore hostile environments, such as in underwater cable, has required that more stringent requirements be imposed on physical properties of the fiber, such as strength. For these more demanding applications as well as for other less demanding ones, it has become increasingly more common to splice optical fibers which have broken, either accidentally, or during appropriate proof testing. Additionally, extremely long lengths of fiber may be obtained by splicing a plurality of lengths which are obtained using current manufacturing techniques. For these and other applications, splicing in which the coating material is removed from end portions of two fibers which are then fused together end to end provides a suitable means for joining the ends of two glass fibers with an acceptable loss. However, the recoating of bared spliced fiber end portions continues as a problem to be overcome, especially while maintaining stringent requirements on dimensional and strength parameters associated with the coated fiber.
A method of recoating spliced end portions of optical fibers is disclosed in U.S. Pat. No. 4,410,561 which issued on Oct. 18, 1983 in the name of A. C. Hart, Jr. The method involves placing the spliced fiber end portions from which the original coating material has been removed and adjacent portions within a cavity in the form of a groove in a split mold. The effective diameter of the groove is somewhat greater than that of the remaining coated portion of each fiber. The fibers are positioned so that only portions of the coated portions of the fibers touch the surface which defines the groove, while the vulnerable, uncoated spliced end portions of the fibers remain suspended and do not contact the groove surface. Then, the mold is covered to enclose the groove and a suitable curable coating material is injected into the groove to recoat the bared, spliced fiber end portions. The recoating material contacts the adjacent originally coated portions of the spliced fibers along substantially radial planes exposed when the original coating material was removed from the end portions and along overlapping portions of the outer surface of the original coating material adjacent to the radial planes. The coating material is then cured to yield a recoated splice section with a transverse cross section which is larger than that of the optical fiber having the original coating material thereon.
This molding process provides a recoated splice; however, steps must be taken to avoid an undesirable number of residual bubbles in the recoating material. The existence of bubbles may lead to stress concentrations when the fiber is handled subsequently. This is particularly undesirable in underwater cables where splices are inaccessible and under stress for many years.
It appears that there are three sources of bubbles. These are air already present in the recoating material, air entrained during the molding process, and bubbles formed during the shrinkage of the recoating material during its cure. The bubbles due to shrinkage tend to be concentrated at the interface between the coating on the unbared fiber portions and the recoating material. This is caused by the pulling away of the recoating material from the coating material on the unbared fiber portions during curing.
Inadequate overlap between the recoating material and the original coating material on the unbared portions of the optical fibers is another problem. Long term integrity of the fiber may be affected by the failure of the recoating material to overlap adequately the original coating material on the portions of the fibers adjacent to the spliced end portions. It may result in the separation of the existing and recoating materials and expose the bare fiber.
Another problem which has surfaced recently relates to the use of optical fibers for tethered vehicles. In these, an optical fiber which is wound on a payoff device and connected to a guidance system is payed off as the vehicle is moved. The payoff device contains a length of precision wound optical fiber.
For tethered vehicles, the winding of the optical fiber on the payoff device must be accomplished in a precision manner. Otherwise, payoff could be disrupted. It has been found that it is difficult to wind a precision package with recoated splices which are made by present techniques. If the cross section of the recoated portion transverse of the longitudinal axis of the optical fiber is not the same as that of the optical fiber as originally coated, the winding pattern on the payoff device in all likelihood is not uniform. This will cause problems in fiber payoff following the launch of the tethered vehicle.
Seemingly, a recoated splice having the same transverse cross section as that of the unspliced fiber has not been attained by the use of prior art methods and apparatus. The transverse cross section of the recoated portion had to be larger to provide overlap of the recoating material with portions of the original coating material adjacent to the recoated end portions, otherwise the necessary adhesion to the original coating material would not be achieved only along the radial planes exposed by the baring of the end portions. When the recoated portion is made larger in a transverse cross section than that of the original coating material, a portion of the recoating material becomes adhered to peripheral portions of the original coated fiber lengths which are adjacent to the beginning of the recoated end portions of the optical fibers and supplements the adhesion along the radial planes.
What is needed and what seemingly is not provided by the prior art is a recoated optical fiber splice which may be used in providing a relatively long length of optical fiber for use in guiding a tethered vehicle, for example. Such a recoated splice must be implemented easily, must have the same transverse cross section as that of the original coated optical fiber and must have integrity of adhesion of the recoating material to the original coating material over a period of time. Also, the sought after methods which are to be used to recoat spliced end portions of optical fibers preferably are such that the formation of bubbles is avoided substantially.